Tuesday, October 16, 2012

West Nile Virus

This has been an exceptional year for West Nile Virus infections in the U.S., especially in Texas where the majority of infections, permanent disability and death have been reported. As always, the most accurate and up-to-date information is available from the Center for Disease Control, whose website is www.cdc.gov. Since this RNA virus is spread almost exclusively by bites from infected mosquitoes, the best way to avoid infection is to minimize exposure to these insects, recalling that they are most likely to bite at dawn and at dusk. Some of the standard instructions include wearing long-sleeved shirts and pants, using mosquito repellent, and not venturing outside at times of maximum exposure. However the Asian tiger mosquito, which has also transmitted the infection in the United States, is a daytime feeder.

The infection is now endemic in the United States in birds that perch. Most birds are just carriers, but crows and robins are often killed by the virus. In fact, the first case reported in the U.S. came from isolation of the virus from a dead crow in New York City, so it is advisable not to handle dead birds but rather to report them to your Board of Health.

The virus was first reported along the Nile River in Uganda in 1937, but now has spread to all tropical and temperate climates throughout the world. It can infect mammals and reptiles, and most hosts show no signs of the disease. There is no vaccine against the disease, and no available anti-viral medicine for it,
so the only treatment for serious cases is support of vital signs in an ICU. It is diagnosed by tests of the blood or of CSF fluid from a spinal tap. It has been spread by blood transfusions, so banked blood is now tested for this disease. It can also be transmitted from pregnant women to the fetus, from nursing women to their children, and by organ transplant. There has been no evidence of people-to-people direct infection.

The majority of humans bitten by infected mosquitoes show no signs of the disease--- 80% have no symptoms at all. Of the remaining 20%, most just show mild symptoms of a viral infection, which can include fever, malaise, rash, swollen lymph nodes, muscle aches and lack of appetite. A few unlucky patients (about 1%) develop infections of the central nervous system, either meningitis or encephalitis, and these may be left with permanent disabilities or have a fatal outcome. The incubation period is 2 to 15 days.

Those at highest risk for CNS infection are the very young, those over 50 years of age, pregnant women, and patients with a weakened immune system, e.g. from cancer chemotherapy, AIDS, or immunosuppression after an organ transplant. Additional risk factors appear to be male sex, diabetes, and hypertension, and some patients may genetically be at higher risk for neurological complications. Some patients may have symptoms for 60 to 90 days, and there have been case reports of chronic fatigue persisting for one to two years after infection.